State changing through very-long-range interactions in high-$n$, $n \ge $ 300, Rydberg-Rydberg collisions

State changing in thermal-energy collisions between atoms in very-high-$n$, $n$ $\ge $ 300, Rydberg states is studied by observing the quantum beats induced by sudden application of a small dc electric field. These so-called Stark beats are shown to be sensitive to angular momentum L and are used to probe the evolution of L during collisions. The data show that, even for impact parameters as large as 50 $\mu $m, collisions lead to rapid L-changing highlighting the long-range nature of the interactions responsible. The corresponding L-changing cross sections are large, 10$^{\mathrm{-4}}$ cm$^{\mathrm{2}}$, and much greater than the ``hard-sphere'' cross sections 4$\pi $r$^{\mathrm{2}}$, where r $\approx n^{\mathrm{2}}$ is the atomic radius. The results also show that measurements of quantum beat amplitudes can provide a valuable complement to selective field ionization when investigating state-changing reactions.